Java Equivalent of C# async/await?

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I am a normal C# developer but occasionally I develop application in Java. I'm wondering if there is any Java equivalent of C# async/await? In simple words what is the java equivalent of:

async Task<int> AccessTheWebAsync()
{ 
    HttpClient client = new HttpClient();
    var urlContents = await client.GetStringAsync("http://msdn.microsoft.com");
    return urlContents.Length;
}

No, there isn't any equivalent of async/await in Java - or even in C# before v5.

It's a fairly complex language feature to build a state machine behind the scenes.

There's relatively little language support for asynchrony/concurrency in Java, but the java.util.concurrent package contains a lot of useful classes around this. (Not quite equivalent to the Task Parallel Library, but the closest approximation to it.)

Is the C# async/Task construct equivalent to Java's Executor/Future , async / await , though, has no equivalent in Java. The key point here is that await someTask is not the same as someFuture.get() . The latter  No, there isn’t any equivalent of async/await in Java – or even in C# before v5. It’s a fairly complex language feature to build a state machine behind the scenes. There’s relatively little language support for asynchrony/concurrency in Java, but the java.util.concurrent package contains a lot of useful classes around this.

Async Await in Java, The cool kids have moved on to using async…await. C# has it. JavaScript has it. And now… and now, via some of the big brains at EA, Java  async / await, though, has no equivalent in Java. The key point here is that await someTask is not the same as someFuture.get (). The latter blocks the executing thread until the future is complete. The former does something completely different.

Check out ea-async which does Java bytecode rewriting to simulate async/await pretty nicely. Per their readme: "It is heavily inspired by Async-Await on the .NET CLR"

Asynchronous Programming in Java, Async instrumentation transforms the code at runtime and rewrites the call to the await method, to behave similarly to using the chain of CompletableFuture. To simplify this, and instruct to run the code asynchronously; C# has introduced async & await keywords. async keyword instructs to the compiler that the declared method has await statements inside, and which has asynchronous operations to execute. async method needs to have an await statement to

async and await are syntactic sugars. The essence of async and await is state machine. The compiler will transform your async/await code into a state machine.

At the same time, in order for async/await to be really practicable in real projects, we need to have lots of Async I/O library functions already in place. For C#, most original synchronized I/O functions has an alternative Async version. The reason we need these Async functions is because in most cases, your own async/await code will boil down to some library Async method.

The Async version library functions in C# is kind of like the AsynchronousChannel concept in Java. For example, we have AsynchronousFileChannel.read which can either return a Future or execute a callback after the read operation is done. But it’s not exactly the same. All C# Async functions return Tasks (similar to Future but more powerful than Future).

So let’s say Java do support async/await, and we write some code like this:

public static async Future<Byte> readFirstByteAsync(String filePath) {
    Path path = Paths.get(filePath);
    AsynchronousFileChannel channel = AsynchronousFileChannel.open(path);

    ByteBuffer buffer = ByteBuffer.allocate(100_000);
    await channel.read(buffer, 0, buffer, this);
    return buffer.get(0);
}

Then I would imagine the compiler will transform the original async/await code into something like this:

public static Future<Byte> readFirstByteAsync(String filePath) {

    CompletableFuture<Byte> result = new CompletableFuture<Byte>();

    AsyncHandler ah = new AsyncHandler(result, filePath);

    ah.completed(null, null);

    return result;
}

And here is the implementation for AsyncHandler:

class AsyncHandler implements CompletionHandler<Integer, ByteBuffer>
{
    CompletableFuture<Byte> future;
    int state;
    String filePath;

    public AsyncHandler(CompletableFuture<Byte> future, String filePath)
    {
        this.future = future;
        this.state = 0;
        this.filePath = filePath;
    }

    @Override
    public void completed(Integer arg0, ByteBuffer arg1) {
        try {
            if (state == 0) {
                state = 1;
                Path path = Paths.get(filePath);
                AsynchronousFileChannel channel = AsynchronousFileChannel.open(path);

                ByteBuffer buffer = ByteBuffer.allocate(100_000);
                channel.read(buffer, 0, buffer, this);
                return;
            } else {
                Byte ret = arg1.get(0);
                future.complete(ret);
            }

        } catch (Exception e) {
            future.completeExceptionally(e);
        }
    }

    @Override
    public void failed(Throwable arg0, ByteBuffer arg1) {
        future.completeExceptionally(arg0);
    }
}

C# vs. Java: 5 Irreplaceable C# Features We'd Kill to Have in Java, If we could have the best of both worlds between Java and C#, what of implementing asynchronous programming, the use of async/await to having equivalent capabilities in asynchronous programming in C# and Java. Here, we are using async programming to read all the contents from the file, so it will not wait to get a return value from this method and execute the other lines of code but it has to wait for the line of code given below because we are using await keyword and we are going to use the return value for the line of code given below..

An Introduction To Java Future, Java Future represents an async work part, similar to async/await in C#. Once the async work part is created, it returns the reference to a Future. In my job I spend a lot of time writing both C# and JavaScript. The code is often related to web applications or services thus usually contain a lot of asynchronous behavior involving http requests…

Could you compare Scala Future and C# async/await?, Scott Danzig, I'm a long-time Java dev that has been using Scala professionally in recent years. Answered Oct 26, 2013 · Author has 2.1k answers and 9.9m  Here, we make a call to the Async.init method in the static block to initialize the Async runtime instrumentation. Async instrumentation transforms the code at runtime and rewrites the call to the await method, to behave similarly to using the chain of CompletableFuture. Therefore, the call to the await method is similar to calling Future.join.

Async/await, In computer programming, the async/await pattern is a syntactic feature of many programming First, the async keyword indicates to C# that the method is asynchronous, meaning that it may use an Pages using deprecated source tags · Pages using deprecated enclose attributes · Articles with example Java code  Select (async ev => await ProcessEventAsync (ev)). Select (t => t. Result). Where (i => i != null). ToList (); But this seems very weird to me, first of all the use of async and await in the select. According to this answer by Stephen Cleary I should be able to drop those. The call to Select is valid. These two lines are essentially identical:

How C# asynchronous programming is different than what you think , Java, Python, and even Javascript are supporting simple syntax as a The async and await keywords have been added after C# as mentioned  C# has it. JavaScript has it. And now… and now, via some of the big brains at EA, Java has it! Yes, Java has a usable async…await construct, without changing the language!

Comments
  • Why this would be nice: Callbacks as our Generations’ Go To Statement by Miguel de Icaza.
  • Java's current solution is to not deal with actual values prefixed with async, but use Future or Observable values instead.
  • There is no equivalent. And it hurts. One more missing feature you need complicated workarounds and libraries for, without ever reaching the same effect as these two, simple words.
  • It's worth noting that for a long time, Java designers have tried to keep the Java bytecode backward compatible with only changes to libraries and Syntatic-sugar around existing features. See the fact that generics don't store run-time type information and lambders are implemented as object implementing an interface. async/await would require very large changes to the bytecode to be possible and therefore I wouldn't expect to see it in java any time soon.
  • @user960567: No, my point is that it's a language feature - it can't be put solely in libraries. I don't believe there's any equivalent scheduled for Java 8 at least.
  • @user960567: You need to distinguish between the version of C# you're using and the version of .NET you're using. async/await is a language feature - it was introduced in C# 5. Yes, you can use Microsoft.Bcl.Async to use async/await targeting .NET 4, but you've still got to use a C# 5 compiler.
  • @rozar: No, not really. There are already multiple options for asynchrony - but RxJava doesn't change the language in the way that C# did. I have nothing against Rx, but it's not the same thing as async in C# 5.
  • @DtechNet: Well there's a lot of JVM machinery which is asynchronous, yes... that's very different from there being actual language features supporting asynchrony though. (There was a lot of asynchrony in .NET before async/await, too... but async/await makes it far easier to take advantage of that.)
  • @Aarkon: I would argue that unless there's explicit language support, the answer is still correct. It's not just a matter of libraries that make scheduling simpler - the whole way the C# compiler builds a state machine is important here.
  • Actually, that's the equivalent of the happy flow. It doesn't cover handling exceptions, finally and others. Including them will make the code much more complex and more prone to errors.
  • This isn't a continuation. This example misses the real purpose of async/await, which is to release the current thread to execute other things, and then to continue execution of this method on the current thread after a response arrives. (This is either necessary for the UI thread to be responsive, or to reduce memory usage.) What this example does is a plain blocking thread synchronization, plus some callbacks.
  • @AleksandrDubinsky I agree with you when you point that the callback may not run on the caller thread. You are right. I disagree about blocking a thread. My updated answer of UPDATED on 25-05-2016 is non-blocking.